1. Field of the Invention
The present invention relates to a liquid crystal display device having a liquid crystal layer placed between substrates, wherein the liquid crystal alignment is substantially horizontal to the substrates when voltage is not applied, and having a twisted structure with a twist angle between 120.degree. and 360.degree. along the direction of the thickness of the liquid crystal layer; and a pair of polarization means disposed to both outsides of the liquid crystal layer.
2. Description of the Background
The liquid crystal display mode predominantly used at present is referred to as the twisted nematic (TN) type in which liquid crystal molecules have a structure twisted by an angle about 90.degree. between a pair of upper and lower substrates, which utilizes the rotation of the plane of polarization due to the liquid crystal and the elimination of such an effect by the application of voltage. This display mode is sufficiently effective for time divisional drive at low rate such as for clocks or pocket electronic calculators but, if it is driven under high time divisional rate for increasing the display capacity, it results in drawbacks such as reduction in the contrast or narrowing in the view angle. This is caused under the time divisional driving at high rate since the ratio of voltage applied to a selection point and a not-selection point approaches 1 and, in order to obtain a high contrast and wide view angle for the display device, it is necessary to reduce the abruptness .gamma. represented by the ratio (V.sub.50 /V.sub.10), which is the ratio of the voltage V.sub.50 where the relative transmittance of the device changes by 50% to V.sub.10 where it changes by 10%, as small as possible.
In the case of the twisted nematic type, the .gamma. value is about 1.13. For reducing the .gamma. value, a method of increasing the twist angle of liquid crystal molecules and deviating the polarization axis from the alignment direction of the liquid crystal has been proposed, which is referred to as SBE (Super Birefringence Effect) mode or STN (Super Twisted Nematic) mode. According to such a method, the .gamma. value can be reduced to less than 1.1 and time divisional driving at high rate of about 1/400 duty ratio is possible.
However, since the coloration due to the birefringence and the change thereof due to the voltage are utilized in such a system, it is theoretically difficult to conduct a black and white display, and the transmission light or reflection light of the liquid crystal cell is colored to cause display on a colored background. In order to eliminate such coloration, it has also been known to superimpose another color-offsetting liquid crystal cell with the opposite twisting direction of the liquid crystal molecules in the STN type liquid crystal cell. However, since two liquid crystal cells are overlaid in this case, it increases the cost. Additionally, the entire thickness and weight are increased and, there is also the drawback that display characters appear to float because of the increase in the distance between the polarization plate and the display liquid crystal layer.
Thus, a need continues to exist for a liquid crystal display device having a reduced thickness and weight as well as having excellent display characteristics.